A motorist's area of vision is very wide and comparatively low. Within this area, only a certain part can be lighted, otherwise oncoming traffic would be blinded or dazzled. Conventional headlight systems in which the light source consists of two optical focusing systems, namely a parabolic reflector (homofocal reflector) and a covering disk, are suitable for this purpose, however they have certain limitations. The corrugations of the covering disk (i.e. its division into groups of individual lenses) are the principal means of obtaining the desired distribution of light, however a square reflector may be used also. The covering disk is often positioned askewed in relation to the optical axis of the headlight for reasons of automobile construction. Another obstacle to appropriate headlight design arises from the reduction in size of the light emitting surface caused by style and aerodynamic considerations.
In order to obtain appropriate lighting densities in spite of these considerations, the so-called PE (polyellipsoid) and DE (three-dimensional ellipsoid) systems have been proposed. These systems are analogous to a slide projector in which the object (slide) is projected via a first collecting lens. The PE and DE systems also include a second collecting lens disposed behind the light source and the covering disk. In contrast to the slide projector, the PE and DE systems must assure a specific light distribution, that is the light/dark borderline is an essential headlight design consideration. So, the systems are provided with a special ellipsoid reflector and a diaphragm (shading blind) disposed in the area of the second focal point of the ellipsoid which creates the desired light/dark borderline.
Up to now, however, the PE and DE systems have proven unreliable in practice despite their optical advantages. The unreliability of the systems is attributed to overheating and adjustment problems. The curved object blind or "Petzval Cup" must be adjusted with the utmost precision. Moreover, this blind blocks a large amount of light energy, which is lost for illumination by conversion to thermal energy. The thermal energy causes deformation and deadjustment of the blind and overheating of the headlight. Further problems are caused by the fact that the edge of the blind, due to its position within the range of the second focal point of the ellipsoid reflector, generates a strong color seam at the light/dark borderline.
DE-OS No. 2461918 discloses a conventional headlight system with a reflector, a projection lens and a diaphragm disposed in between the described projection lenses. FIG. 5 shows an embodiment without a diaphragm in which a light/dark borderline is formed by a section of an elliptical concentrating reflector whose edge extends parallel to the incandescent filament and not by providing a reflector which is symmetrical to the light source. However, the basic requirement of the device according to claim 1 of the patent is that the surface of the exit pupil must not exceed two square centimeters in any direction. The embodiment of FIG. 5 does not comply with this requirement because direct light enters the diffusion lens from the horizontal lamp. For the embodiment of FIG. 5 to meet this requirement, a diaphragm must be provided as illustrated in each of the other drawings. Therefore, FIG. 5 must be considered to be incomplete because the disclosure must always be considered in its entirety.
DE-OS No. 3241826 discloses a PE system including an elliptical reflector having two focal points (F.sub.1, F.sub.2) a collecting lens, and a refractive diaphragm disposed at the second focal point F.sub.2. The focal point of the collecting lens coincides with F.sub.2. According to this publication, the object is to determine critical zones on the elliptical reflector and to render those zones nonreflecting or less reflecting by surface treatment or changing the zone's shape.
FR-A-2501333 discloses an optical focusing system entirely different from the instant vehicle lighting system. In the disclosed optical focusing system, the covering disk is optional. The projection lens generates the image pattern directed to the reflector. According to the reference, the projection lens, a purely diffusing lens, is followed by a diaphragm and deviation mirrors directing light against the reflector from where it is reflected toward the covering disk. Accordingly, a section-wise different curvature of the projection lens in respect to the image pattern generated by the headlight is not provided because the projection lens of the device does not project the light outside of the headlight.
GB-PS No. 1570805 discloses a headlight with a horizontally split concave reflector which allows light to project out the sides of the headlight at an angle to the main forward light ray. A bi-convex lens section covers a zone proximal the optical axis of the incandescent bulbs used in this headlight. The lens section receives only part of the emitted light. Moreover, the lens section has two identical curved convex zones. The covering disk may have a lens-like construction, but only covers the zone proximal the optical axis.
DE-OS No. 3200796 discloses dimmer lights comprising a parabolic reflector, a lamp and a dimming cap placed between the lamp and the reflector. The dimming cap includes a concave face oriented toward the light source and has adjacent areas of different curvatures.